as they say live and learn, sucks, but is what it is now!
I am going to take a guess that if this atv had mud in its air box, it was an ATV that was driven in the deep mud/water, and h=odds are HIGH it was also an atv that was sunk 1 time or maybe more!
and thus sold to someone that didn;'t know any better!
and now IMO< if you wanted to make things right and be sure is GOOD for the future
time to pull motor , tear down, clean, inspect and make sure all that needs fixing get fixed!
do this, and ALSO< make sure whole fuel system is clean ans you will have a reliable atv again your deal maybe on buying it will go out the window, but is what it is now
for future info
when buying an atv, there are basics to look at, as ell as before heading out on a NEW to YOU atv
a check list of sorts
this is, check fuel for water, oil too, check air box, make sure air filter is CLEAN before heading out, check OIL level too make sure its at correct level and looks CLEAN, doesn't smell of fuel or being burnt!
look for excessive mud built up on things, to show it was rather abused over cared for
a GOOD owner washes things after playing in mud
cleans/inspects air filter often!
many seller just power wash the outside to make pretty and BAIL a beat up other wise machine to an unsuspecting buyer!
suck atv's are common these days, and then sold saying they haven't been !
so you need to TRY and find signs of it before buying!
smarter sellers do more cover up! making it harder LOL
but buying used is a gamble and always will be